Dídia Covas
Instituto Superior Técnico
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Publication
Featured researches published by Dídia Covas.
Journal of Hydraulic Research | 2005
Dídia Covas; Ivan Stoianov; João F. Mano; Helena M. Ramos; Nigel Graham; Cedo Maksimovic
A state-of-the-art mathematical model has been developed to calculate hydraulic transients in pressurized polyethylene (PE) pipe systems. This hydraulic transient solver (HTS) incorporates additional terms to take into account unsteady friction and pipe-wall viscoelasticity. Numerical results obtained were compared with the classic waterhammer solution and with experimental data collected from a PE pipe-rig at Imperial College (London, UK). Unlike the classical model, the developed HTS is capable of accurately predicting transient pressure fluctuations in PE pipes, as well as circumferential strains in the pipe-wall. The major challenge was the distinction between frictional and mechanical dynamic effects. First, the HTS was calibrated and tested considering these two effects separately: if only unsteady friction was considered, a major disagreement between collected data and numerical results was observed; when only the viscoelastic effect was considered, despite the good agreement between data and numerical results, the calibrated creep function depended on the initial flow rate. In a second stage, the combination of these dynamic effects was analysed: creep was calibrated for laminar flow and used to test the solver for turbulent conditions, and a good agreement was observed. Finally, the HTS was tested using creep measured in a mechanical test, neglecting unsteady friction, and a good agreement was obtained.
Journal of Hydraulic Research | 2004
Dídia Covas; Ivan Stoianov; Helena M. Ramos; Nigel Graham; Cedo Maksimovic
The mechanical behaviour of the pipe material determines the pressure response of a fluid system during the occurrence of transient events. in viscoelastic pipes, typically made of polyethylene (pe), maximum or minimum transient pressures are rapidly attenuated and the overall pressure wave is delayed in time. this is a result of the retarded deformation of the pipe-wall. this effect has been observed in transient data collected in a high-density pe pipe-rig, at imperial college (london, uk). several transient tests were carried out to collect pressure and circumferential strain data. the pipe material presented a typical viscoelastic mechanical behaviour with a sudden pressure drop immediately after the fast valve closure, a major dissipation and dispersion of the pressure wave, and transient mechanical hysteresis. the creep-function of the pipe material was experimentally determined by creep tests, and, its order-of-magnitude was estimated based on pressure-strain data collected from the pipe-rig. a good agreement between the creep functions was observed. creep tests are important for the characterization of the viscoelastic behaviour of pe as a material; however, when pe is integrated in a pipe system, mechanical tests only provide an estimate of the actual mechanical behaviour of the pipe system. this is because creep depends on not only the molecular structure of the material and temperature but also on pipe axial and circumferential constraints and the stress-time history of the pipe system.
Journal of Hydraulic Research | 2004
Helena M. Ramos; Dídia Covas; Alexandre Borga; Dália Loureiro
The current study focuses on the analysis of pressure surge damping in single pipeline systems generated by a fast change of flow conditions. A dimensionless form of pressurised transient flow equations was developed, presenting the main advantage of being independent of the system characteristics. In lack of flow velocity profiles, the unsteady friction in turbulent regimes is analysed based on two new empirical correctivecoefficients associated with local and convective acceleration terms. A new surge damping approach is also presented taking into account the pressur e peak time variation. The observed attenuation effect in the pressure wave for high deformable pipe materials can be described by a combination of the non-elastic behaviour of the pipe-wall with steady and unsteady friction effects. Several simulations and experimental tests have been carried out, in order to analyse the dynamic response of single pipelines with different characteristics, such as pipe materials, diameters, thickness, lengths and transient conditions.
Journal of Hydraulic Research | 2008
Anton Bergant; As Arris Tijsseling; John P. Vítkovský; Dídia Covas; Angus R. Simpson; Martin F. Lambert
This two-part paper investigates key parameters that may affect the pressurewaveform predicted by the classical theory ofwater-hammer. Shortcomings in the prediction of pressure wave attenuation, shape and timing originate from violation of assumptions made in the derivation of the classical waterhammer equations. Possible mechanisms that may significantly affect pressure waveforms include unsteady friction, cavitation (including column separation and trapped air pockets), a number of fluid–structure interaction (FSI) effects, viscoelastic behaviour of the pipe-wall material, leakages and blockages. Engineers should be able to identify and evaluate the influence of these mechanisms, because first these are usually not included in standard water-hammer software packages and second these are often “hidden” in practical systems. Part 1 of the two-part paper describes mathematical tools for modelling the aforementioned mechanisms. The method of characteristics transformation of the classical water-hammer equations is used herein as the basic solution tool. In separate additions: a convolution-based unsteady friction model is explicitly incorporated; discrete vapour and gas cavity models allow cavities to form at computational sections; coupled extended water-hammer and steel-hammer equations describe FSI; viscoelastic behaviour of the pipe-wall material is governed by a generalised Kelvin–Voigt model; and blockages and leakages are modelled as end or internal boundary conditions
Journal of Hydraulic Research | 2008
Anton Bergant; As Arris Tijsseling; John P. Vítkovský; Dídia Covas; Angus R. Simpson; Martin F. Lambert
This two-part paper investigates parameters that may significantly affect water-hammer wave attenuation, shape and timing. Possible sources that may affect the waveform predicted by classical water-hammer theory include unsteady friction, cavitation (including column separation and trapped air pockets), a number of fluid–structure interaction effects, viscoelastic behaviour of the pipe-wall material, leakages and blockages. Part 1 of this two-part paper presents the mathematical tools needed to model these sources. Part 2 of the paper presents a number of case studies showing how these modelled sources affect pressure traces in a simple reservoir-pipeline-valve system. Each case study compares the obtained results with the standard (classical) water-hammer model, from which conclusions are drawn concerning the transient behaviour of real systems
Urban Water Journal | 2004
Dídia Covas; Ivan Stoianov; Helena M. Ramos; Nigel Graham; Cedo Maksimovic; David Butler
This paper analyzes the dynamic effects of pipe wall viscoelasticity on hydraulic transients. These effects have been observed in transient data collected from two polyethylene (PE) pipe systems. The first is a 270 m pipeline, 50 mm diameter, at Imperial College London, and the second is the worlds longest experimental PE pipeline, 1.3 km long, 110 mm diameter, buried underground at Thames Water Utilities (London, UK). A mathematical model has been developed to calculate hydraulic transients in polyethylene pipe systems based on the assumption that the viscoelastic behaviour of pipe walls is linear. An additional term has been added to the continuity equation to describe the retarded deformation of the pipe wall and the resulting governing equations are solved by the Method of Characteristics. The numerical results are compared with both the classic elastic solution and with collected transient data. Good agreement between numerical results for the viscoelastic solution and observed data was obtained by fitting the creep function J(t). Unlike classic water hammer analysis, the developed mathematical model is capable of accurately predicting transient pressures in polyethylene pipes and the circumferential strains in the pipe walls.
Journal of Hydraulic Research | 2012
Alexandre Kepler Soares; Dídia Covas; Nelson Carriço
The research focuses on the analysis of transient cavitating flow in polyethylene pipes, which are characterized by a viscoelastic–rheological behaviour. A hydraulic transient solver incorporating the description of the rheological–mechanical behaviour of these pipes and the cavitating pipe flow has been developed. Both the discrete vapour cavity model and the discrete gas cavity model have been used to describe transient cavitating flow. These assume that discrete air cavities are formed at fixed pipe sections and consider a constant wave speed in the pipe reaches between the cavities. An experimental programme was conducted in a pipe-rig composed of a high-density polyethylene (HDPE) pipeline. Numerical results have been compared with collected data resulting in a good agreement. Effects related to the viscoelasticity of HDPE pipes and to the occurrence of vapour pressures during the transient events are also discussed.
Water Science and Technology | 2012
Miguel Agostinho Cardoso; M. Santos Silva; Sérgio T. Coelho; Marcela Almeida; Dídia Covas
Water services are a strategic sector of large social and economic relevance. It is therefore essential that they are managed rationally and efficiently. Advanced water supply and wastewater infrastructure asset management (IAM) is key in achieving adequate levels of service in the future, particularly with regard to reliable and high quality drinking water supply, prevention of urban flooding, efficient use of natural resources and prevention of pollution. This paper presents a methodology for supporting the development of urban water IAM, developed during the AWARE-P project as well as an appraisal of its implementation in four water utilities. Both water supply and wastewater systems were considered. Due to the different contexts and features of the utilities, the main concerns vary from case to case; some problems essentially are related to performance, others to risk. Cost is a common deciding factor. The paper describes the procedure applied, focusing on the diversity of drivers, constraints, benefits and outcomes. It also points out the main challenges and the results obtained through the implementation of a structured procedure for supporting urban water IAM.
Urban Water Journal | 2014
C. Apollonio; Dídia Covas; G. de Marinis; Angelo Leopardi; Helena M. Ramos
New experimental results obtained from a high density polyethylene (HDPE) pipeline for transient conditions are presented and discussed. Transient pressure data at different pipe sections were collected. Peculiar features were observed, which are typical of viscoelastic pipes, in particular, a significant damping of extreme pressures was observed in comparison to what is typically obtained for metal pipes. Physical constraints of the pipe are important to describe some vibrations observed in the system. The viscoelastic transient solver developed by Covas et al. (2005) was used to carry out numerical simulations of experimental tests. The comparison of numerical results with experimental data has shown that the viscoelastic model accurately predicts observed transient pressures. Additional considerations are carried out about the calibrated creep function for this test case in comparison with the ones obtained in previous researches.
Water Science and Technology | 2012
Nelson Carriço; Dídia Covas; M. Céu Almeida; João P. Leitão; Helena Alegre
The aim of this paper is to compare sorting and ranking methods for prioritization of rehabilitation interventions of sewers, taking into account risk, performance and cost. For that purpose multiple criteria decision-aid (MCDA) methods such as ELECTRE TRI for sorting and ELECTRE III for ranking are applied in a real case-study and the results obtained are compared. The case study is a small sanitary sewer system from a Portuguese utility located in the metropolitan area of Lisbon. The problem to investigate is the prioritization of the sewer candidates for rehabilitation. The decision maker (a panel group of specialists) has chosen five assessment measures: water level and maximum flow velocity (hydraulic performance indices), sewer importance and failure repair cost (collapse-related consequences of failure) and the risk of collapse. The results show that the outcomes from ELECTRE III are easier to understand than those from ELECTRE TRI method. Two different sets of weights were used, and the sorting and ranking results from both methods were found to be sensitive to them. ELECTRE TRI method is not straightforward as it involves technical parameters that are difficult to define, such as reference profiles and cut levels.